The present invention relates generally to a successive sheet feed mechanism for use in a printing device such as a printer or the like. In particular, the present invention relates to a successive sheet feed mechanism that is able to feed successive sheets in the forward or reverse direction when the width of the successive sheets increase or decrease due to a change in temperature, humidity or the like.
Generally, a conventional successive sheet feed mechanism includes a pair of tractors with each tractor having a pin belt. The tractor is designed to mesh the holes, respectively formed along the two side portions of each sheet of paper, with the pins provided on the pin belts to feed the successive sheets of paper in the forward or reverse direction.
If the width of the successive sheets of paper increase or decrease due to a change in temperature, humidity or the like, and the position of the pair of tractors (that is, pin belts) is fixed, then the holes in the successive sheets will not be properly aligned with the pins in the pin belts, thereby resulting in the successive sheets of paper not being correctly fed in the forward conveying or reverse conveying direction.
Japanese Patent Laid Open Publication Nos. 1-271344 (Heisei) and 3-216447 (Heisei), respectively, describe proposed sheet feeding mechanisms that attempt to solve the heretofore mentioned problem.
FIG. 1 depicts the sheet feed mechanism described in Japanese Patent Laid Open Publication No. 1-271344 of Heisei.
As shown in FIG. 1, a frame member 3 is slidably mounted onto a drive shaft 1 and a guide shaft 2. A tractor 4 is installed in frame member 3 and is slidably mounted onto drive shaft 1 and guide shaft 2. Tractor 4 is forced in direction X1 by a pair of springs 5 between frame member 3 and tractor 4. Holes 6a are respectively formed on the two side portions of each successive sheet 6. The sheet feed mechanism has an endless pin belt, which is not shown in FIG. 1, with a plurality of pins 4a disposed thereon. Pins 4a are to be aligned and meshed with holes 6a to convey the paper in a forward or reverse direction. A cover plate 4b is provided for covering each successive sheet 6 loosely by holding the two portions of each successive sheet 6 on opposed sides of holes 6a between the pin belt and cover plate 4b. Cover plate 4b has an elongated hole 4c that allows pins 4a to extend therethrough. Furthermore, a lock mechanism 7 selectively secures frame member 3 to guide shaft 2 so that frame member 3 can be prevented from being slidable. Although FIG. 1 depicts only the left sheet feed mechanism, the right sheet feed mechanism is identical to the mechanism in FIG. 1 except to the extent that the right sheet feed mechanism does not include frame member 3, and is also fixed to drive shaft 1 and guide shaft 2.
Reference is now made to FIGS. 2a and 2b to describe the operation of aligning successive sheet 6 to the thus constructed sheet feed mechanism. First align the holes on the right side of successive sheet 6 on the right tractor (not shown) which is fixed to drive shaft 1 and guide shaft 2. Then, after lock mechanism 7 has been released, frame member 3 and tractor 4 are slidable to thereafter bring pins 4a of tractor 4 into alignment with holes 6a of successive sheet 6. Cover plate 4b is then closed, loosely trapping the side portions of successive sheet 6 between the pin belt and cover plate 4b. Then, frame member 3 is pulled in the direction of arrow c, as shown in FIG. 2b, thereby producing a clearance S between frame member 3 and tractor 4. Frame member 3 can be locked in place by lock mechanism 7 and successive sheet 6 is properly set.
In this arrangement, successive sheet 6 always has applied to it a tensile force caused by spring 5, causing tautness in each sheet in the widthwise direction.
Therefore, even if the width W of successive sheet 6 increases or decreases due to a change in temperature, humidity or the like, the increase or decrease is compensated by the sliding of tractor 4 along drive shaft 1 and guide shaft 2. In this manner, each successive sheet 6 can be conveyed in the forward or the reverse direction.
The conventional sheet feed mechanism employing the structures described above has the drawback that it is necessary to provide frame member 3, which makes the mechanism very complicated.
FIG. 3 depicts a second prior art embodiment of a sheet feed mechanism as described in Japanese Patent Laid Open Publication No. 3-216447 (Heisei). This embodiment attempts to eliminate the above-mentioned problem of an overly complicated mechanism. In this embodiment, reference numeral 8 designates generally left and right tractors mounted on a drive shaft 1 and guide shaft 2, the right tractor being shown with its cover plate 8d open, the left tractor being shown with its cover plate 8d closed. Each tractor 8 includes a main body generally indicated at 8a and a pin belt 8b. Main body 8a is used as the above-mentioned frame member 3 (FIG. 1). A pin belt 8b is slidable with respect to main body 8a on shaft 8e. A spring 8c is positioned between main body 8a surrounding each shaft 8e and pulleys (not shown) over which pin belt 8b is extended are respectively slidably mounted on shafts 8e, one of the pulleys being operatively coupled to drive shaft 1.
The conventional sheet feed mechanisms employing the structures described above have the following problems.
First, if a proper clearance S is not formed between tractor 4 and frame member 3, as shown in FIGS. 1 and 2, or between pin belt 8b and main body 8a, as shown in FIG. 3, the proper alignment for the successive sheets of paper cannot be obtained. For example, if in FIG. 2b, clearance S is set too small, tractor 4 will not be able to extend the amount necessary to compensate for the width of successive sheet 6 if the increase in the width of successive sheet 6 is greater than clearance S. On the other hand, if clearance S is set too large, then tractor 4 will not be able to contract the necessary distance to compensate for the decrease in width of successive sheets 6. As shown in FIG. 2a, if clearance S is set at zero because of operator misoperation, tractor 4 cannot extend at all. When tractor 4 cannot follow the extension or contraction of successive sheets 6, then the successive sheets cannot be conveyed properly.
Further, in the conventional sheet feed mechanisms described above, if the sheet is not set properly and the clearance S is caused to vary, it is extremely difficult to obtain the proper operation. To obtain a desired operation, it is necessary to adjust the sheet precisely to provide for a proper clearance S. This precise adjustment requires complicated setting mechanisms and operator skill.
Furthermore, while frame member 3 is being adjusted in the direction of an arrow c against the tensile force of the spring, lock mechanism 7 must be essentially simultaneously adjusted, which makes it difficult for some to use the mechanism.
Accordingly, it is desired to provide an improved successive sheet feed mechanism having a construction which will allow for easy adjustments and settings and will also allow successive sheets to be conveyed even if the sheets shrink or expand.